I've the privilege to discuss about ideal caudal finnagae with some
of our outstanding local betta hobbyist and here's what our kind friend "bettabetter" from Arofanatics has diligently produce from the IBC judging standards manual so that all may benefit.

Please find the following

Standard on the caudal fin:

Caudal Fin
In singletail bettas, the ideal shape is a semicircle that spreads to a perfect 180 degrees. Due to proper selection of breeders, fin ray branching, care, conditioning, and fin spread this is an elusive ideal to breed for and maintain. Because of this, some classes will sometimes not have specimens with this ideal caudal among them.

In such classes, with all other things equal, the most symmetrical types having the widest spread and those with the least faults as described in the general fault guide and the special standards will have the best chance of placing. Caudal fins that exceed 180